This study is to determine the effects of rifabutin, (a rifamycin analog of rifampin) ethambutol, (an ethylenediamine with selective anti-mycobacterial activity) or both on the pharmacokinetics of clarithromycin, (an extended spectrum macrolide antibiotic) in subjects with advanced AIDS and Mycobacterium avium complex (MAC) infection. It is also to explore the relationship between plasma levels of clarithromycin and short-term changes in quantitative colony counts of MAC in peripheral blood.